1.3 V Inorganic Sequential Redox Chain with an All-Anionic Couple 1–/2– in a Single Framework

The relatively low symmetry of [3,3′-Co(1,2-C2B9H11)2]− ([1]−), along with the high number of available substitution sites, 18 on the boron atoms and 4 on the carbon atoms, allows a fairly regioselective and stepwise chlorination of the platform and therefore a very controlled tuning of the electrochemical potential tuning. This is not so easily found in other systems, e.g., ferrocene. In this work, we show how a single platform with boron and carbon in the ligand, and only cobalt can produce a tuning of potentials in a stepwise manner in the 1.3 V range. The platform used is made of two icosahedra sharing one vertex. The E1/2 tuning has been achieved from [1]− by sequential chlorination, which has given potentials whose values increase sequentially and linearly with the number of chloro groups in the platform. [Cl8-1]−, [Cl10-1]−, and [Cl12-1]− have been obtained, which are added to the existing [Cl-1]−, [Cl2-1]−, [Cl4-1]−, and [Cl6-1]− described earlier to give the 1.3 V range. It is envisaged to extend this range also sequentially by changing the metal from cobalt to iron. The last successful synthesis of the highest chlorinated derivatives of cobaltabis(dicarbollide) dates back to 1982, and since then, no more advances have occurred toward more substituted metallacarborane chlorinated compounds. [Cl8-1]−, [Cl10-1]−, and [Cl12-1]− are made with an easy and fast method. The key point of the reaction is the use of the protonated form of [Co(C2B9H11)2]−, as a starting material, and the use of sulfuryl chloride, a less hazardous and easier to use chlorinating agent. In addition, we present a complete, spectroscopic, crystallographic, and electrochemical characterization, together with a study of the influence of the chlorination position in the electrochemical properties.


■ INTRODUCTION
Redox reactions are key for life both in nature, 1 principally in respiration 2 and photosynthesis, 3 and in any device where electrons are the means to store, release, or generate energy. 4−11 In most of the redox reactions in industry to produce bulk materials or compounds, no fine-tuning of the reduction or oxidation power is sought. However, this is not so when it is necessary to ensure the synergy with surrounding materials or compounds that can be affected by an excess of oxidizing or reducing power. E°tuning of man-made redox-reversible systems is largely based first on metals and second in ligands, 12−19 Notice from this sentence that we emphasize metal-based redox-reversible systems. We will not deal with nonmetal-based systems because, for the case of boron clusters, these are derived from [CB 11 H 12 ] −20 or [B 12 H 12 ] 2− . 21 It is important to point out that nature succeeds in getting a wide range of potentials with few metals, few coordinating elements, and few ligands for the primary coordination spheres but requires the involvement of one or two extra spheres of influence to modulate E°. 15 Some robust metal-containing scaffolds have been developed on which to tune the redox potential by the sequential addition of electron-donor or -acceptor groups or π acceptors. Some of the more studied scaffolds are due to ferrocene, 22,23 or metal complexes, most commonly ruthenium, of polypyridyl ligands, e.g., bipyridine, 2,2′-bipyrimidine, 2,2′-bipyrazine, terpyridine, phenanthroline, and others. 24 Their common factor is that they are usually outer-sphere electron-transfer octahedral complexes. A quite representative example of the type of E°tuning in these complexes is given by the ferrocene [FeC 10 H 10−x Cl x ] chloro derivatives for which brusque, the opposite of stepwise, numbers of chloro units exist, e.g., 10, 5, 2 and 1, which result in brusque E 1/2 values, versus ferrocenium/ferrocene (Fc + /Fc) of 1.24, 0.77, 0.31, and 0.17 V, respectively. Still, nearly 1 V has been tuned on the same platform. 25,26 All of these complexes are positively charged, e.g., [Fe(C 5 Cl 5 ) 2 ] + or [Ru(bpy) 3 ] 2+ . Indeed, despite the fact that ligands are either negative or neutral, very few chemically stable and robust anionic complexes are available ready for E°tuning. One could consider the couple [Fe(CN) 6 ] 3−/4− , or the polyoxometallates (POMs), e.g., Keggin [XW 12 O 40 ] n− ; 27 however, these are difficult to tune, although efforts are being made for POMs. 28 Thus, anionic metal-containing scaffolds that allow easy tuning with a wide span of voltages are not common. Also, what could be the advantage of using anionic scaffolds? In our opinion, if the reduced form of the redox couple is negative, it will have an increased tendency to release an electron, and if the oxidized form is negative, it will have less appetence for an electron. Plus, this can be easily spotted with the iodide/ triiodide (I − /I 3 − ) redox couple in dye-sensitized solar cells (DSSCs) in which both the oxidized and reduced partners are negative. 10,29,30 Cobalt-31 and copper-based electrolytes, 32 thiolate/disulfide, 33 Fc/Fc + , 34 hydroquinone/benzoquinone derivatives, 35 and the redox couple TEMPO/TEMPO +36 all either have a partner whose charge is zero, have a partner with a positive charge, or have both partners with a positive charge. The success of a DSSC relies on the electrons preferring to move through the external circuit to meet the counter electrode rather than the electrons on the TiO 2 surface recombining with the dye or oxidized electrolyte. 37 We have already indicated that it is not simple to have metalbased robust redox couples based on a single scaffold that allow for a wide range of potentials. In this work, we show that this is becoming possible with the anionic cobaltabis-(dicarbollide) [3,3′-Co(1,2-C 2 B 9 H 11 ) 2 ] − scaffold (abbreviated as [1] − ). This cluster displays interesting electrochemical and biological properties that have been thoroughly studied. 38 49 and electroactive electrolytes among others.
The relatively low symmetry of [1] − , along with a high number of available substitution sites, allows a fairly regioselective and stepwise chlorination of the platform and therefore a very controlled tuning of the sought-after property, in this case potential tuning. Such characteristics are not easily found in other systems. On the other hand, a higher symmetry, as in many closo clusters, leads more easily to persubstitution but with more difficulty to a step-by-step process. 50 We present here the three highest chlorinated species of [1] − , which will be named [Cl 8 -1] − , [Cl 10 -1] − , and [Cl 12 -1] − , corresponding to the number of chloro substituents on the scaffold, which span the voltages from −1.75 V for [1] − to −0.49 V for [Cl 12 -1] − , versus Fc + /Fc in sequential chlorination steps, and very remarkably with very good electrochemical purity and high yield in simple one-pot reactions ( Figure 1). This series is the widest range of sequentially tunable potentials on a single metal-containing anionic platform available today. Also, the range of potentials possible can be extended much further by keeping the same platform, changing the metal from cobalt to iron.

■ RESULTS AND DISCUSSION
Synthesis. Since the synthesis of the first halogenated derivative of COSAN, the hexabromocobaltabis-(dicarbollide), 51 many strategies have been devised to develop halo derivatives of [1] − . The most advanced since that date is the development of iodo derivatives of [1] − , whose methodology requires the buildup of molecules from the components, so the synthesis of [1,5,6,10-I 4 -7,8-C 2 B 9 H 10 ] − , followed by their complexation with CoCl 2 , yields [3,3′-Co(8,9,12,10-I 4 -1,2-C 2 B 9 H 7 ) 2 )] − , which is the halo derivative of cobaltabis-(dicarbollide) with the highest number of halo substituents produced until now. 52,53 Chlorine gas was the most popular chlorinating agent for [1] − , 54−56 with [3,3′-Co(8,9,12-Cl 3 -1,2-C 2 B 9 H 8 ) 2 53 being the highest chlorinated [1] − obtained as a pure compound since   57 Sulfuryl chloride is less hazardous, cheaper, and easier to handle than chlorine gas and has been successfully applied as a chlorinating agent in organic chemistry. 58,59 To achieve chlorination in boron clusters, solubilization of the cesium and tetramethylammonium (the most common) salts of the different boron clusters was needed, but these are not fully soluble in sulfuryl chloride. In 2010, a step forward was achieved by mixing acetonitrile with sulfuryl chloride to increase the solubility. This new method allowed the synthesis of pure [B 12 Cl 12 ] 2− , 60 and afterward, the same methodology was used to obtain the hexachloroferrabis(dicarbollide) 61 and tetrachloro- 62 and hexachlorocobaltabis(dicarbollide). 63 However, we did not succeed in going beyond with this mixture, even after several days of refluxing and repositioning of acetonitrile and sulfuryl chloride. Therefore, the combination of sulfuryl chloride with acetonitrile was not considered to be the best option. Instead, we have gone with H [1], which is more soluble in neat sulfuryl chloride. 64 Because sulfuryl chloride has a relatively low boiling point, 69°C, we aimed at increasing the reaction pressure to lower reaction times and increasing the reaction temperature. Stainless steel autoclaves, even lined with Teflon, were proven not to be suitable because extensive damage was caused by the generated chlorine gas at the autogenous pressure induced by external heating at 120°C. We then moved to thick-walled glass pressure tubes with Ace-Thred poly(tetrafluoroethylene) bushing and FETFE O-ring. The O-rings were replaced every four experiments. These proved to be adequate for our purposes.
The reaction of H [1] with an excess of SO 2 Cl 2 (650 equiv) in an Ace pressure tube at 70°C for 4 days is a convenient route to synthesizing the octachloro derivative presented as an isomeric mixture of [3,3′-Co (4,7,8,9,12- (Figure 2a). This turned out to be the maximum chlorination degree achievable by this method. Attempts to increase the chlorination degree by increasing the reaction time to a few weeks or using higher temperatures did not lead to notable amounts of [Cl 9 -1] − or [Cl 10 -1] − ) derivatives. It was then proven that a convenient and easy method leading to constitutionally, although not isomerically, pure [Cl 8 -1] − was available. To increase the number of chloro groups in the molecule, a Lewis acid such as AlCl 3 (1 equiv) was added to the reaction mixture, and this turned out to be the determining factor in obtaining a higher chlorination degree, leading to the production of [3,3′-Co(4,7,8,9,10,12-Cl 6 -1,2-C 2 B 9 H 5 ) 2 ] − ([C 12 -1] − ), the highest imaginable chlorinated redox-reversible couple ( Figure 2b). In addition, the amount of SO 2 Cl 2 was optimized to control the chlorination degree, leading to [3,3′-Co(4,7,8,9,12-Cl 5 -1,2-C 2 B 9 H 6 ) 2 ] − ([Cl 10 -1] − ). Thus, while the synthesis of [Cl 12 -1] − requires a huge excess of SO 2 Cl 2 (650 equiv), the synthesis of [Cl 10 -1] − needs less chlorinating agent (65 equiv). The methodology consists of a mixture of 0.1 and 65 equiv of AlCl 3 and SO 2 Cl 2 , respectively, with 1 equiv of H [1] in an Ace pressure tube at 70°C for 2 days. Then the tube is open, and the solvent is removed under reduced pressure. Then, 0.1 and 65 equiv more of AlCl 3 and SO 2 Cl 2 , respectively, are added to the solid reaction mixture, and the tube is closed again and is heated at 70°C for 2 more days ( Figure 2b).
Characterization. All new compounds were characterized by 1 H, 1 H{ 11 B}, 13 C{ 1 H}, 11 B, 11 B{ 1 H} NMR, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) in the negative mode, elemental analysis, IR, and X-ray diffraction. The complete spectral information and crystallographic data can be found in the Supporting Information (SI). The IR spectra give us a qualitative analysis of the reaction by monitoring of the B−H band around 2600 cm −1 . In addition, a comparison of the Fourier transform infrared (FTIR) spectrum of Na [1] with the spectra of  Inorganic Chemistry pubs.acs.org/IC Article percentage of less than 10% of the side products corresponding to compounds with one chloro plus or less (see the SI). 56 The study of the NMR spectra, together with X-ray diffraction, led us to unveil the exact positions of the chloro substituents. Suitable single crystals of [NMe 4 ][Cl 8 -1] and [NMe 4 ][Cl 10 -1] were obtained by slow evaporation in acetone; for Cs[Cl 12 -1], crystals were obtained in CH 2 Cl 2 , and as far as we are concerned, they are the highest halogenated derivatives of metallacarborane ever crystallized ( Figure 3). As a result of all of these contacts, the three structures revealed the less common conformations in the cobaltabis-(dicarbollide) derivatives. 66 (Figure 5a,b). In addition, these intermolecular interactions are so strong that they persist in solution,   Figure 6). Fortunately, separation of the mixture was possible thanks to the different polarities of the isomers. In particular, the isomer [Cl 8β -1] − was very insoluble in chloroform, leading to an isomeric pure product that could be analyzed by 1 H, 11 (Figure 7 and Table 1), most likely due to the mixture of isomers, which causes slightly different potentials, and because of the overlap of the two traces, a thicker signal is found.

Structures and Intermolecular
As a rule of thumb, it was considered that each new chloro added to the structure contributes +0.12 V to the E 1/2 (Co III / Co II ) value. 56 Figure 8 shows that indeed the increment of E 1/2 (Co III /Co II ) (ΔE 1/2 ) is quasilinear, except for the first ([Cl 2 -1] − ) and last ([Cl 12 -1] − ) points, showing a considerable deviation from the expected values. This accounts for the importance of the chlorinated position, a phenomenon previously observed in the iodinated derivatives. 68 It has been demonstrated that the anionic [1] − cluster is a global 3D aromatic system 69 with a negative charge delocalized all over the system. 70 Considering that the chloro substituent is an electron-withdrawing group, each additional chloride makes   (Table 1). In addition, this experiment demostrates the hypothesis of a broad ΔmV    was heated in an Ace pressure tube at 70°C for 2 days. When the reaction has finished, the closed tube was left to cool at room temperature. The tube was opened, and the solvent was removed under reduced pressure. A total of 20.5 mg (0.15 mmol) of AlCl 3 was added again, and the mixture was dissolved in 8 mL of SO 2 Cl 2 in the same Ace pressure tube. The reaction was heated at 70°C for another 2 days. When the reaction had finished, the closed tube was left to cool at room temperature. The product was purified following the same treatments as those used with the compound [NMe 4 ][Cl 8 -1]. The solid was filtered and dried, and 924 mg of a red solid corresponding to the product NMe 4 [Cl 10 -1] was obtained (yield: 90%). 1